Transmitting data of all types is a key feature of economic and cultural life today.
The objective of transmission techniques, which are tending more and more towards digital signal transmission, is to reduce transmission speeds by coding and compression and then decoding and decompression to avoid overloading the transmission networks and optimise their use as a result, whilst preserving high transmission quality.
More specifically in the field of digital signal transmission, for example digital audio/video signals, the quality of the transmitted signal is essentially a concept based on subjective analysis. Accordingly, in the case of a digital audio signal or a digital audio/video signal, the subjective quality of this decoded-coded sound signal is a quality that is evaluated on the basis of subjective tests. These subjective tests essentially consist in assigning a value score to the decoded-coded signal by asking a group of listeners to assess the quality.
Various psycho-acoustic models have been developed as part of the techniques used to implement digital compression on digital audio signals. These models have also enabled objective perceptual measuring methods to be applied as a means of evaluating the subjective quality of coded digital audio signals.
All of these objective perceptual measuring methods operate on a same principle, which consists in comparing the decoded-coded sound signal with the original signal. The original signal is the digital audio signal prior to coding and compression, also referred to as the reference signal. In order to apply the above-mentioned methods, it is therefore necessary to have a reference signal, which must be as finely synchronised with the digital audio signal to be tested as possible. Constraints of this type can be readily dealt with by simulation, for example, so that a coding process can be evaluated logically, or when a digital audio signal or a digital audio/video signal is emitted and broadcast, at the stage when this latter is emitted in order to evaluate the objective quality of the coded signal transmitted. In this latter instance, the original signal is necessarily available and it is possible to determine the delay caused by the process used to generate the coded signal, which can be applied as a means of re-synchronising this coded signal, which will be the signal tested, and the reference signal. However, this is not always the case when the digital audio signal or digital audio/video signal is received during the transmission. To date, the only way of assessing the quality of a broadcast signal in the case of a digital audio signal is to listen to it in order to judge the subjective quality.
The objective of this invention is to remedy the above-mentioned drawback so that a process can be applied to a broadcast digital signal immediately on reception in order to evaluate the transmission quality and, more specifically in the case of a digital audio signal or a digital audio/video signal, take perceptual measurements.
This objective is achieved by applying a method of evaluating the quality of a digital signal broadcast as a transmission from an initial signal, this method consisting in generating a reference signal representative of this initial signal and broadcasting the digital signal and the reference signal jointly. At the reception stage, this method consists in synchronising the broadcast digital signal received and the broadcast reference signal received and computing a distance value, denoting the variance from identity, separating the broadcast digital signal received from the broadcast reference signal received. This distance value can be used as a criterion for assigning a quality value to the digital signal on reception.
The present invention can be applied in the field of digital signal transmission by Hertzian or satellite channel, particularly in the case of audio signals and/or audio/video signals such as television or radio signals.